Inorganic gels are well known and have long been used for various purposes, for example, for the adsorption of condensible vapors from gaseous carriers and for catalytic purposes either as a catalyst itself or as a component thereof or as a carrier. The most widely used of these inorganic gels in the dried condition is that of silica with or without the addition of other gelatinous materials such as alumina.
Inorganic gels in combination with a chromium compound have been found to be particularly useful as catalysts for carrying out reactions involving hydrocarbons, especially olefins such as ethylene, as disclosed in U.S. Pat. No. 3,887,494. It is known that the melt index and thus the molecular weight of an olefin polymer is responsive to the average pore diameter of the catalyst support. By increasing the average pore diameter of the catalyst support, therefore, there is achieved an increase in the melt index of the polymer prepared in the presence of a catalyst containing such support.
The manner in which a catalyst or support to be utilized in a catalytic composition is prepared can influence the average pore diameter of the support, and hence the melt index of the polymer prepared in the presence of the catalyst composition. Methods of preparing catalysts and inorganic hydrogels used in preparing catalyst compositions are well known in the art, such as those disclosed in U.S. Pat. No. 3,887,494; U.S. Pat. No. 2,978,298; U.S. Pat. No. 2,503,913; and U.S. Pat. No. 2,429,319.
In the preparation of an inorganic catalyst support from a hydrogel, e.g., a silica-containing hydrogel, an important step is the removal of water from the hydrogel or the drying of the hydrogel. The step is important in that the manner in which the water is removed has a great bearing upon the size of the pores of the catalyst support. One method of drying hydrogel is to heat the hydrogel at a high temperature in order to evaporate the water. A problem, however, is that the walls of the pores in the hydrogel are in a plastic condition during drying and, as a result of the high surface tension of the water, the capillary pores are pulled together as the water is evaporated, thereby leaving small pores in the dried hydrogel.
The prior art has somewhat overcome this problem by removing the water from the hydrogel through repeated washings with an organic compound or by azeotropic distillation. The organic compounds utilized are usually aliphatic monohydric alcohols or ketones, as disclosed in U.S. Pat. No. 2,429,319; U.S. Pat. No. 2,503,913; and U.S. Pat. No. 2,978,298. U.S. Pat. No. 2,503,913 also discloses the use of a water-immiscible liquid such as kerosene with or without a small amount of surface-active agent.
Surprisingly, however, it has been discovered that the use of oxygen-containing organic compounds selected from the group consisting of polyhydric alcohols, the mono- and dialkyl ethers of alkylene glycols and poly(alkylene) glycols and mixtures thereof, saves in the cost of the preparation of the dried hydrogels in that a costly azeotropic process is not used. Other unexpected results are that the pore size of the dried gels is not adversely affected by drying and hence the melt index of the polymers prepared over the catalyst compositions is improved over that obtained through the use of monohydric alcohols or ketones.
An object of the present invention, therefore, is to produce an improved dried inorganic hydrogel to be used as a catalyst support for olefin polymerization processes.
A further object of the present invention is to produce a dried inorganic hydrogel with increased pore volume.
An additional object of the present invention is to prepare a dried, chromium compound containing inorganic hydrogel that will yield a more effective catalyst, i.e., improve the polymer melt index of the polymer prepared in the presence of the catalyst.
A still further object of the present invention is to provide an improved process for polymerizing olefins.
Another object of this invention is to provide an improved catalyst for the polymerization of ethylene.
Other objects, aspects, and the several advantages of this invention will be apparent to those skilled in the art upon a study of this disclosure and the appended claims.